WHY THE OUTPUT AMPLITUDE LEVEL IS SO HIGH..!

[santom]

Hi all,
I am trying to build a second order Delta sigma modulator in cadence tool.I am supplying a 500 mV as my input to my first integrator along with negative feedback from the comparator section of the delta sigma modulator.

The output swing of the first integrator is pretty much reasonable as it is in the same range of the input voltage.But for the second integrator, the ouput swing voltage is ramping like to a very high level(400 KV).I am not able to figure out the thing going on there.

Can anyone help me in that..Valuable suggestions are most welcome.

Santom

 

[FvM]

Fortunately, 400 kV are present in simulation only!

To answer more seriously, you have either an input overrange or incorrect coefficients in your design. There should be a negative feedback path to the input, that prevents the second integrator from saturation. You also may want to supplement your simulation model with a voltage limitation according to a real circuit, at least if you want to study overrange effects as well.

 

[santom]

Hi,
Thanks for the reply.There is a negative feedback from the output to the second integrator and still I am getting the response in this way.As long as the co efficients are concerned, they are pretty OK I guess.

If you have the cadence tool and feel like to checking it out, I will send you the related files.I am stuck up with this issue and couldnt go further.I really need a positive outcome asap as it is too demotivating..!

 

[FvM]

Unfortunately, I don't have it. But I guess, the behaviour isn't specific to this simulator. However, if your integrator basically has a limiting characteristic, but you experience these ridiculous kV voltages anyway, it may be a convergence problem of the simulator, that should be reported as such. It may be still caused by a not meaningful circuit, e. g. positive instead of negative feedback, but also by a combination of device and transient simulation parameters. The tools user guide should have detailed suggestion how to handle it.